1. Field of the Invention
This invention relates generally to a new composition of matter, and more specifically to new high oxidation state metal persulfido complex that can be used for generating hydrogen from water, in one embodiment the high oxidation state metal being molybdenum.
2. Brief Description of the Related Art
Owing to issues of climate change and accelerating global energy demands, the search for viable carbon-neutral sources of renewable energy is amongst the foremost challenges in science today. One such alternative is hydrogen, which can potentially be used as a clean replacement for fossil fuels in many applications, including transportation in cars, buses, trucks, trains, and airplanes. It can further be used in fuel cells for powering mobile devices such as lap-top computers and cell phones, as well as for meeting power requirements in buildings and industry. Many industries also use hydrogen as a reactant. One example is the Haber-Bosch process that produces ammonia, which currently relies on steam reforming of natural gas or liquefied petroleum for the production of hydrogen. This is expensive, environmentally unsustainable (based on finite resources of fossil fuel and produces carbon dioxide and hydrogen sulfide, two major atmospheric pollutants) and necessitates removal of sulfur which deactivates the catalyst used for ammonia production. Hydrogen is also used as a reducing agent for metal ores, for the production of hydrochloric acid and as a hydrogenating agent for unsaturated fats and oils.
In this context, where hydrogen has emerged as an attractive candidate for a clean, sustainable fuel as well as a precursor to many essential compounds, an intense interest in creating artificial systems that utilize earth-abundant catalysts for efficient hydrogen production from water has developed. A major quest of this renewable energy research is the search for efficient catalysts for the production of hydrogen from water which rely on cheap, earth-abundant elements.
Hydrogenase enzymes possessing earth-abundant iron and/or nickel cofactors have been found to catalytically evolve H2 from neutral aqueous solutions at its thermodynamic potential, with turnover frequencies of 100-10,000 mols H2/mol catalyst per second. However, the large size and relative instability of these enzymes under aerobic, ambient conditions has led to the search for well-defined molecular complexes outside the biological milieu that can produce H2 from water. Although many examples of air- and moisture-sensitive synthetic iron-sulfur clusters have provided insight into hydrogenase structure and reactivity, they catalyze proton reduction from acids in organic solvents at fairly negative potentials of −0.9 to −1.8 V vs. the SHE (Standard Hydrogen Electrode). Metal complexes that evolve H2 at more positive potentials still require organic acids, additives, and/or solvents. As such, the creation of earth-abundant molecular systems that produce H2 from water with high catalytic activity and stability remains a significant basic scientific challenge.
Water electrolysis has also been achieved through the use of precious metal catalysts (e.g. platinum, palladium) and purified water, as well as at elevated temperatures, all of which makes the process expensive. In a search for lower cost alternatives, in related case PCT US2010/048405, a low cost alternative to the precious metal catalysts is described. More particularly, these low cost metal complexes are salts wherein the cation comprises a PY5 metal-oxo ion. (As used herein, PY stands for pyridine and PY5 indicates the presence of five pyridyl rings). The positively charged cations of those compositions were described by the general formula [(PY5W2)MO]2+, wherein PY5W2 is (NC5XYZ)(NC5H4)4C2W2, and M a high oxidation state metal. In one disclosed embodiment, the metal was molybdenum (Mo), which in terms of cost is about 74 times lower than the cost of platinum, the current preferred catalyst for hydrogen production. In other disclosed embodiments, W, X, Y, and Z of the general formula were described as being selected from the group comprising H, R, a halide, CF3, or SiR3, where R is an alkyl or aryl group. The substitutions at the X, Y, and Z positions were further disclosed as being either the same or different. Finally the group attached to the quaternary carbon at the W position was described as either being hydrogen, methyl, a higher alkyl or aryl group or any one of the halogens F, Cl, Br and I, CF3 or SiR3. The accompanying negative ions (i.e. the counter anion) for these metal-oxo salt compositions said to be any one of a number of anions, including a halide such as Cl−, I−, or PF6−, CF3SO3−, and so forth. The exact composition of the anion was not found to be significant, as it was not found to play a significant role in the water to hydrogen reaction. These pentapyridine ligand complexes are semi rigid, and in their salt form easily dissolve in water.
These organo metal-oxo complexes catalytically generated hydrogen from water at neutral pH. In one embodiment, the organo metal-oxo complex was an organo molybdenum-oxo complex, which in experiments successfully generated hydrogen for at least 3 days, with a turnover frequency (TOF) of at least 1.47 million mol H2/mol catalyst per hour (i.e., 408 mol H2/mol catalyst per second) and a turnover number (TON) of 105 million mol H2/mol catalyst. Moreover, this same molecular system was used to evolve H2 from seawater, the earth's most abundant source of protons.
The discovery of a molecular metal-oxo catalysts, and more particularly molybdenum-oxo catalysts for generating hydrogen from water without use of additional acids and/or organic co-solvents established a new chemical paradigm for creating reduction catalysts that are highly active and robust in aqueous media. Importantly, that system employed an inexpensive, earth-abundant metal to achieve catalytic H2 evolution from neutral buffered water or even seawater, maintaining long-term activity with TOF values in excess of 400 or more mol H2/mol catalyst per second and TON values of 105 million mols H2/mol catalyst. An overpotential of between −0.6 V to −1.0 V at the cathode lead to an efficiency of 67%-55% respectively for the cell, assuming that the rest of the cell was operated at ideal efficiencies.
Notwithstanding the success of the MO based catalyst, and more particularly the MoO based catalyst, there remains the need for a low cost and efficient catalyst for generating hydrogen gas from water, which is stable, low cost, and can produce hydrogen at even lower over-potentials than that currently realized with MO.